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Horse Power for Steam used.                  627

Such a method of reckoning horse power is convenient when
deciding boiler capacity and heating surface.    Then :

Steam per minute )      (I. H. P.) 33,000 + 144^^: AN
in cubic ft.

Steam per minute \              ,. I. H. P.      cAN } _  _

-------~~r;--------- I = 229-16               -i-----— L Df Lq

in Ibs.           j          *         prN         rV   )         *

where V is specific volume at the higher temperature, Df the
diagram factor (p. 772), and Lq the liquefaction factor (See
Appendix //, p. 884). If steam per brake horse power be desired
the value B. H. P. —77 may be inserted instead of I. H. P., where
r\ is the mechanical efficiency. Willans' important law connecting
steam consumption and H. P. is given in Appendix II., p. 892.

In the above formulae p is mean effective pressure per square
in. At p. 625, this quantity is estimated in terms of initial
pressure. If then it be required to know the volume of steam
used, in terms of the initial pressure, it is only necessary to
substitute the value at p. 625 for/. (See Appendix III., p. 931.)

General idea of the various forms of Steam Engine.
—The steam engine is a prime mover designed for converting
heat into work by allowing steam to expand behind a working
piston. Sometimes the work need only be of a reciprocating
nature; while in other cases, and this by far the greater number,
rotative motion is required, and the crank and connecting rod, or
some similar appliance is then employed, as fully set out at
pp. 486 to 496. Sometimes also a rotative shaft is introduced,
with a fly-wheel to assist in maintaining regular reciprocating
motion, where that only is needed, or perhaps to work the valves,

The Beam Engine^ though almost obsolete, has served and is
serving much useful purpose, and a few of its applications will
therefore be described. In Fig. 626, A is a Cornish pumping-
engine, a being the cylinder, e the working beam, and f the
pump-rod passing down the pit-shaft Steam is that known as
' low pressure,' having only a few pounds7 pressure above the
atmosphere; and there are three drop valves, ft, c, d, for its
distribution, called respectively the steam, equilibrium, and
exhaust valves. The last passes the steam into the condenser g,